High-speed triangular pattern phase-shifting 3D measurement based on the motion blur method

Ultra-small, low-cost, and simple-to-control PSP projector based on SLCD technology

Demand for ultra-small, inexpensive, and high-accurate 3D shape measurement devices is growing rapidly, especially in the industrial and consumer electronics sectors. Phase shifting profilometry (PSP) is a powerful candidate due to its advantages of high accuracy, great resolution, and insensitivity to ambient light. As a key component in PSP, the projector used to generate the phase-shifting sinusoidal fringes must be ultra-small (several millimeters), low-cost, and simple to control. However, existing projection methods make it difficult to meet these requirements simultaneously. In this paper, we present a modern technique that can be used to fabricate the desired projector. A specifically designed device based on segmented liquid crystal display (SLCD) technology is used to display the projected patterns, and a cylindrical lens is used as the projection lens. The SLCD device can display four sets of specific filled binary patterns, each yielding a sinusoidal fringe, and all four sinusoidal fringes satisfy the four-step phase shift relation. 3D shape measurement experiments verify the performance of the projector. Considering that the size of SLCD devices can be reduced to a few millimeters, the proposed technique can be easily used to manufacture ultra-small, low-cost, and simple-to-control PSP projectors.

Projected feature assisted coarse to fine point cloud registration method for large-size 3D measurement

Fringe projection profilometry has gained significant interest due to its high precision, enhanced resolution, and simplified design. Typically, the spatial and perspective measurement capability is restricted by the lenses of the camera and projector in accordance with the principles of geometric optics. Therefore, large-size object measurement requires data acquisition from multiple perspectives, followed by point cloud splicing. Current point cloud registration methods usually rely on 2D feature textures, 3D structural elements, or supplementary tools, which will increase costs or limit the scope of the application. To address large-size 3D measurement more efficiently, we propose a low-cost and feasible method that combines active projection textures, color channel multiplexing, image feature matching and coarse-to-fine point registration strategies. Using a composite structured light with red speckle patterns for larger areas and blue sinusoidal fringe patterns for smaller ones, projected onto the surface, which allows us to accomplish simultaneous 3D reconstruction and point cloud registration. Experimental results demonstrate that the proposed method is effective for the 3D measurement of large-size and weak-textured objects.

Spatially resolved birefringence measurements with a digital micro-mirror device

We demonstrate a novel technique to measure spatially resolved birefringence structures in an all-digital fashion with a digital micro-mirror device (DMD). The technique exploits the polarization independence of DMDs to apply holographic phase control to orthogonal polarization components and requires only a static linear polarizer as an analyzer for the resulting phase shift polarization measurements. We show the efficacy of this approach by spatially resolving complex polarization structures, including nano-structured metasurfaces, customized liquid crystal devices, as well as chiral L-Alanine and N-Acetyl-L-cystein crystals. Concentration dependent measurements of optical rotation in glucose and fructose solutions are also presented, demonstrating the technique's versatility. Unlike conventional approaches, our technique is calibration free and has no moving parts, offers high frame rates and wavelength independence, and is low cost, making it highly suitable to a range of applications, including pharmaceutical manufacturing, saccharimetry and stress imaging.

Encoding technology of an asymmetric combined structured light for 3D measurement

Sinusoidal phase-shifting symmetrically combined with cyclic code is one of the most important encoding methods in the field of 3D measurement. Due to the modulation of the object surface and the influence of the noise of the image acquisition system, the periods of the cyclic code and the sinusoidal phase-shifting in the intensity image do not coincide completely, and they lead to large absolute phase decoding errors near the cycle boundaries, which are called cycle dislocation errors. In order to eliminate these errors in principle, the concept and method of region encoding for four-step sinusoidal phase-shifting are proposed, and the sinusoidal phase-shifting is combined with cyclic code asymmetrically. Under the premise that the cyclic code and the region code change at different times, the cycle dislocation error is reduced from one cycle of cyclic code to one pixel by the dual constraint of cyclic code and region code. The simulation measurement results of 3 ds max and the physical measurement results show that the asymmetric combination encoding method effectively eliminates the cycle dislocation errors; the maximum measurement error is reduced by an order of magnitude, and the root mean square measurement error is reduced by 70%.

Online fringe pitch selection for defocusing a binary square pattern projection phase-shifting method

A three-dimensional (3D) shape measurement system using defocusing binary fringe projection can perform high-speed and flexible measurements. In this technology, determining the fringe pitch that matches the current projection's defocus amount is of great significance for an accurate measurement. In this paper, we propose an online binary fringe pitch selection framework. First, by analyzing the fringe images captured by the camera, the defocus amount of projection can be obtained. Next, based on analysis of the harmonic error and camera noise, we establish a mathematical model of the normalized phase error. The fringe pitch that minimizes this normalized phase error is then selected as the optimal fringe pitch for subsequent measurements, which can also lead to more accuracy and robust measurement results. Compared with current methods, our method does not require offline defocus-distance calibration. However, it can achieve the same effect as the offline calibration method. It is also more flexible and efficient. Our experiments validate the effectiveness and practicability of the proposed method.

3D Face Profilometry Based on Galvanometer Scanner with Infrared Fringe Projection in High Speed

Structured light 3D shape metrology has become a very important technique and one of the hot research topics in 3D face recognition. However, it is still very challenging to use the digital light projector (DLP) in a 3D scanner and achieve high-speed, low-cost, small-size, and infrared-illuminated measurements. Instead of using a DLP, this paper proposes to use a galvanometer scanner to project phase-shifted fringes with a projection speed of infrared fringes up to 500 fps. Moreover, the measurement accuracy of multi-frequency (hierarchical) and multi-wavelength (heterodyne) temporal phase unwrapping approaches implemented in this system is analyzed. The measurement accuracy of the two methods is better than 0.2 mm. Comparisons are made between this method and the classical DLP approach. This method can achieve a similar accuracy and repeatability compared to the classical DLP method when a face mask is measured. The experiments on real human face indicate that this proposed method can improve the field of 3D scanning applications at a lower cost.

High-speed real-time 3D shape measurement based on adaptive depth constraint

Stereo phase unwrapping (SPU) has been increasingly applied to high-speed real-time fringe projection profilometry (FPP) because it can retrieve the absolute phase or matching points in a stereo FPP system without projecting or acquiring additional fringe patterns. Based on a pre-defined measurement volume, artificial maximum/minimum phase maps can be created solely using geometric constraints of the FPP system, permitting phase unwrapping on a pixel-by-pixel basis. However, when high-frequency fringes are used, the phase ambiguities will increase which makes SPU unreliable. Several auxiliary techniques have been proposed to enhance the robustness of SPU, but their flexibility still needs to be improved. In this paper, we proposed an adaptive depth constraint (ADC) approach for high-speed real-time 3D shape measurement, where the measurement depth volume for geometric constraints is adaptively updated according to the current reconstructed geometry. By utilizing the spatio-temporal correlation of moving objects under measurement, a customized and tighter depth constraint can be defined, which helps enhance the robustness of SPU over a large measurement volume. Besides, two complementary techniques, including simplified left-right consistency check and feedback mechanism based on valid area, are introduced to further increase the robustness and flexibility of the ADC. Experimental results demonstrate the success of our proposed SPU approach in recovering absolute 3D geometries of both simple and complicated objects with only three phase-shifted fringe images.